This invention relates to a process for producing 2,6-naphthalene dicarboxylic acid (hereinunder referred to as 2,6-NDA) which is useful as a raw material for high quality polyester.
2,6-NDA has been worthy of notice as a raw material for high quality polyester having excellent thermal resistance, mechanical strength, dimensional accuracy and the like. Development of a commercial process for producing 2,6-NDA has been much in demand.
Prior arts on the production of 2,6-NDA are generally of the following three types.
(1) Processes for producing 2,6-NDA which comprise oxidizing 2,6-dimethyl naphthalene in the presence of a catalyst comprising a heavy metal and a bromine compound are disclosed in U.S. Pat. No. 3,856,855 and Japanese Patent Publication (Kokai) No. 34153/1973. In these processes, it is difficult to separate the raw material, 2,6-dimethyl naphthalene from the dimethyl naphthalene mixtures, and the amount of 2,6-dimethyl naphthalene separated is not sufficient.
(2) A process for producing 2,6-NDA which comprises oxidizing 2,6-diisopropyl naphthalene in the presence of a catalyst comprising Co and Mn is disclosed in Japanese Patent Publication (kokai) No. 89445/1985. This process involves use of an excessive amount of the catalyst. So this process is not proper from industrial point of view.
(3) Processes for producing 2,6-NDA which comprise oxidizing a 2-alkyl-6-acyl naphthalene in the presence of a catalyst containing Co and Br or Co, Mn and Br are disclosed in Japanese Patent Publication (kokai) Nos. 61946/1987 and 67048/1987 and U.S. Pat. No. 4,764,638. However, the yield of 2,6-NDA is insufficient in these processes.
Advantageously, 2-alkyl-6-acyl naphthalenes can be easily obtained in a high yield by Friedel Crafts Reaction between commercially available 2-methyl naphthalene and acetyl fluoride or butyryl fluoride. Therefore, 2-alkyl-6-acyl naphthalenes have became remarkable as a raw material for 2,6-NDA.
The present inventors have produced crude 2-methyl-6-butyryl naphthalene from 2-methyl naphthalene, propylene, carbon monoxide and hydrogen fluoride and purified the crude 2-methyl-6-butyryl naphthalene. The inventors have attempted the production of 2,6-NDA from 2-methyl-6-butyryl naphthalene by oxidation under conditions which were known in the prior art. However, high yields of 2,6-NDA cannot be obtained from a 2-alkyl-6-acyl naphthalene in the prior arts.